The present invention relates to an apparatus for determining the vapor pressure of a fluid. The apparatus is especially useful in the direct determination of Reid Vapor Pressure (hereinafter sometimes referred to as RVP) of a volatile petroleum product. The invention more particularly is directed to an apparatus and method for producing accurate and reproducible RVP measurements of volatile, normally liquid, hydrocarbon fuels.
Automotive gasoline is a product which is manufactured in large quantities by petroleum refiners to a set of refiner's specifications. Automotive gasolines also must comply with certain set specifications which are established by various State regulations which include limited variations for geographic locations and seasonal changes. The specifications for automotive gasoline are established in American Society for Testing Materials (ASTM) specifications designated D-439. These specifications establish the required properties of automotive gasolines for ground vehicles, and they describe various characteristics of automotive gasolines which may be used in a wide range of operating conditions. The specifications allow automatic variation to the gasoline manufacturer to meet the requirements of seasonal changes in temperature, depending upon the locality in which the product is to be used. This is done by providing five volatility classes and differentiating the use of the volatility classes according to the months of the year and the geographical location in the United States where the product is expected to be used.
The ASTM specification D-439 establishes a schedule of changes within the volatility classes for seasonal and geographical differences. From the D-439 specification a gasoline manufacturer may determine the volatility characteristics of the gasoline that may be produced and sold in any particular geographic location at a particular season of the year. Reid Vapor Pressure is one of the volatility measurements established in the D-439 specification for automotive gasoline.
In the manufacture of motor gasolines, the manufacturer blends into the product a number of different hydrocarbon materials. The end product that is produce d from the blending operation must conform to the D-439 specifications for an automotive gasoline. The specifications may include anti-knock performance, vapor-liquid ratio, Reid Vapor Pressure, a distillation characteristic, corrosion, existent gum, sulfur, and other specific requirements for the respective geographic locations.
Reid Vapor Pressure of motor gasoline is measured by another separate ASTM standard D-323 which prescribes how a sample of motor fuel is to be handled and establishes the absolute procedure that must be used in determining Reid Vapor Pressure using a specific air chamber referred to as a vapor tester bomb. RVP measurements in accord with the D-323 procedure take a substantial period of time (about 25 minutes, plus equipment preparation time) and the measurements are subject to variations due to variations in sample handling, test conditions, and equipment preparation all of which lead to imprecise test results. Such delay and imprecision in the measurement of RVP by the D-323 apparatus is incompatible with the continuous process of producing blended motor fuels in the manufacturing of gasoline products.
It is therefore believed to be advantageous to develop a substantially completely automated procedure for handling motor gasoline samples and for measuring the RVP of these samples. Preferably, the procedure can be accomplished with a self-contained measurement device that will produce a direct measurement of the actual RVP of the product being produced. With such a procedure and device a motor fuel manufacturer may have a quick and precise indication of the RVP of the product being produced in a motor gasoline blending operation.
There is but one way to measure RVP directly and that is to use D-323. The present invention duplicates the essential conditions of D-323; thus it is designed to measure RVP directly without the need for corrections. Previous automated devices of which we are aware do not duplicate D-323, therefore their results must be correlated with RVP. A measurement which must be correlated with RVP includes a correlation error and the resultant measurement is therefore likely to be less accurate than a direct measurement.
The following specification describes an apparatus, and a method for operating the apparatus, for providing a rapid, accurate, reproducible and dependable measurement of Reid Vapor Pressure--RVP.